Air Quality Guidelines Global Update 2005 - World Health ...

398
Health effects
AIR QUALITY GUIDELINES
Effects on experimental animals
The acute effects of sulfur dioxide at ambient concentrations may be conveniently
studied by short-term exposure of human volunteers. The effects of longterm
exposure to sulfur dioxide can, however, be studied only in experimental
animals. At concentrations in excess of 28.6 mg/m 3 (10 ppm), prolonged exposure
has been shown to produce damage to the epithelium of the airways. This
may be followed by epithelial hyperplasia, a dose-related increase in goblet cells
and hypertrophy of the submucosal glands. These changes are similar to those
seen in chronic bronchitis in humans; prolonged exposure of rats and dogs to
sulfur dioxide has been used to produce models of this disease.
In addition to the morphological changes mentioned above, slowing of ciliary
transport of mucus has also been demonstrated, although only on exposure to
high (858 mg/m 3 or 300 ppm) concentrations of sulfur dioxide. Effects were seen
on exposure to much lower concentrations of sulfuric acid. Disruption of ciliary
function has recently been shown on exposure of explanted human bronchial
tissue to a combination of 572 μg/m 3 (200 ppb) sulfur dioxide and 752 μg/m 3
(400 ppb) nitrogen dioxide.
Studies in a range of species have shown that exposure to sulfur dioxide produces
bronchoconstriction. Studies in guinea-pigs show responses in the same
concentration range as has been shown to affect asthmatic volunteers (about
715 μg/m 3 or 0.25 ppm).
Studies by Amdur (2) have demonstrated that the effects of sulfur dioxide may
be enhanced by simultaneous exposure to ultrafine particles. In particular, zinc
oxide (2.5–5 mg/m 3 ) and sulfur dioxide (2860 μg/m 3 or 1 ppm) led to decreased
lung volumes and carbon monoxide diffusing capacity in guinea-pigs. These
changes were correlated with oedema and damage to epithelial and endothelial
cells. Responses increased as the concentration of zinc oxide was raised. The formation
of sulfuric acid on the surface of the ultrafine particles and the particles’
delivery to the distal lung have been put forward as explanations of these effects.
Re-analysis of air pollution data collected in London in the 1960s has shown a
stronger relationship between concentrations of sulfuric acid aerosol and effects
on health than with concentrations of black smoke or sulfur dioxide. These findings,
and the association between high concentrations of sulfur dioxide, particles
and water vapour during the London smogs prior to the 1970s, suggest that the
health effects may have been caused by sulfuric acid delivered on the surface of
fine and ultrafine particles.